Erythema responses in Caucasians and light-, medium-, and dark-complexioned American Negroes were compared. Minimal erythema dose (MED) in various skin shades was correlated with melanosome size, quantity, density, and distribution. No minimal erythema response was found to be typical of Negro skin, but a spectrum of responses was found, depending upon the degree of pigmentation. Dark Negro skin had an MED 33 times that of the Caucasian, contained the greatest density of pigment, and had the largest melanosomes, all singly dispersed. Caucasian skin contained the smallest quantity of pigment and the smallest melanosomes, most of which were contained within melanosome complexes. Measurements for the lightand medium-complexioned Negroes were intermediate. The determining factor as to whether melanosomes are individually dispersed or are taken into complexes is size-dependent. The upper size limit for melanosomes found within complexes in this study was about 0.35μ.